The present invention relates to field effect transistor devices and more particularly, to such devices containing differentially doped surface and subsurface structures which provide reduced electric field strength levels at the interface boundary between the gate oxide and the semiconductor body.
One of the most useful and advantageous electrical devices is the field effect transistor (FET). Very large scale integrated (VLSI) circuits have been created using metal-oxide-semiconductor (MOS) field effect devices. These so-called MOSFET devices take their name from the structure of the device in the vicinity of the gate. The gate electrode typically comprises a metal layer insulated by a silicon oxide layer from a doped semiconductor layer beneath the oxide. In the present invention, the problem addressed is the trapping of hot or energetic electrons in the gate oxide region, particularly in those regions adjacent to the device drain.
Decreased cost and improved performance provide an impetus for miniaturizing metal oxide semiconductor devices even further. Continuing advances in lithography and etching will permit even greater reductions in device size. Unfortunately, for reasons such as the compatibility with existing systems, the power supply voltages used in such devices usually fails to decrease with increasing circuit density. As a result of this violation of ideal scaling, electric field strength levels within the device increase as geometric distances shrink. High field effects within MOS devices include carrier mobility reduction and channel hot electron instability in MOSFETs, particularly n-channel MOSFETS or, NMOSFETs. Electrons flowing from the source to drain regions of an n-channel FET gain energy from the lateral electric field component. This component of electric field is parallel to the interface between the silicon semiconductor and the silicon oxide insulating layer beneath the gate electrode. Energetic electrons tend to surmount the silicon/silicon-oxide energy barrier and are trapped within the oxide gate insulator or generate undesirable interface states by mechanisms which are not yet fully understood.
Hot or energetic electrons from the channel current have a tendency to acquire sufficient energy to surmount the barrier between the semiconductor and the gate oxide and as a result, are injected into the oxide. Subsequent trapping of the injected electrons tends to cause device instabilities such as threshold-voltage shift and transconductance degradation. Accordingly, it is highly desirable that hot-electron resistant MOSFETs be devised to prevent such instabilities. These instabilities impose undesirably severe limitations on the further miniaturization of MOSFET devices in VLSI circuits.
While improvements in hot electron effect problems can be partially effected by increasing the thickness of the gate oxide, this is an undesirable option because it reduces gate control effects and results in slower operating devices.
It is seen that the hot electron problem is certainly undesirable and may eventually limit the minimum gate length for MOSFETs. Although improving the insulator quality with respect to hot electron trapping effects may be possible, most efforts at improving channel hot electron reliability have focused on reducing the lateral electric field strength within the FET channel. Such attempts include graded source/drain junctions, double-diffused source/drain regions, and an oxide sidewall spacer adjacent to the gate electrode and oxide. All of these methods reduce the lateral field strength by making the n.sup.+ -p drain-channel transition less abrupt. The instant inventors have found that the oxide sidewall spacer can increase the maximum safe operating voltage by as much as 2 volts. A maximum gain of 1.5 volts per 1,000 angstroms of spacer length is possible. It is additionally noted that any method or structure which is directed at relieving the hot electron effect problem should also be compatible with present VLSI semiconductor processing steps.